1. Field of the Invention
The present invention relates to radar systems. More specifically, the present invention relates to bistatic radar systems.
2. Description of the Related Art
In a monostatic radar system, the transmitter and the receiver are co-located. In a bistatic radar architecture, the transmitter and receiver are substantially separated. In addition, both the transmitter and the receiver may be mounted on either fixed or moving platforms. Bistatic radar is therefore distinguished from monostatic radar where the transmitter and receiver are mounted on the same platform and move together.
When the transmitter and/or the receiver is in motion, the reflected signal will be Doppler frequency shifted as a function of relative motions and positions of the transmitter and receiver and the position of the target. In effect, the motions and positions of the transmitter and receiver paint the landscape with a spatial Doppler frequency gradient. If the target is also in motion, its reflected signal will have an additional Doppler shift dependent on the target""s velocity vector.
With conventional airborne monostatic radar systems, it is often difficult to detect xe2x80x9cslow movingxe2x80x9d targets (targets which generate low Doppler shifts) in surface clutter due to the clutter Doppler spread resulting from the varying Doppler shifts of the terrain illuminated by the radar transmit beam. This can mask the target return if its Doppler shift is within the mainbeam clutter Doppler spread (the clutter Doppler shift over a range cell). For monostatic radar systems, the clutter Doppler spread depends on the motion of the radar platform relative to the transmit look direction. This motion is usually fixed due to system constraints and is not free to be optimized. For bistatic radar systems, however, the clutter Doppler spread is due to the combined motion of both the transmitter and receiver which are mounted on separate platforms. Because the motions of the transmitter and receiver are independent, they are free to be optimized in order to control the bistatic clutter Doppler spread. This process is known as xe2x80x9cclutter tuning.xe2x80x9d
According to conventional analysis, when the motion of the receiver is in the opposite direction to that of the transmitter, the receiver""s Doppler frequency shift for a particular terrain object, situated equidistant from the transmitter and receiver, partially or completely cancels the transmitter""s Doppler shift. This cancellation condenses the clutter Doppler spectrum into a narrowed frequency spectrum, while a moving target""s Doppler frequency offset remains nearly the same with respect to the terrain center frequency. The returns from relatively slow moving targets may therefore be found outside the clutter spectrum and may become highly observable. Thus, by suitably opposed motions of the transmitter and receiver for the foregoing restricted scenario, a bistatic radar system can become much more effective at detecting slowly moving targets than an equivalent monostatic radar wherein slow moving targets are lost in clutter. The practical problem is how to oppose the motion of the transmitter and receiver so as to achieve substantial clutter condensation, when the target location relative to the transmitter and receiver is not constrained in any manner.
One widely accepted method of flying the transmitter and receiver is to have them both on the same side of the target, but flying in opposite directions (e.g. one clockwise and the other counterclockwise). This solution gives good range resolution, but it reduces the duration of continuous observation of a given target area since the aircraft rapidly fly out of the region where standard analysis predicts good performance for detecting moving targets.
Thus, while bistatic radar provides a theoretically interesting class of solutions to the problem of detecting targets which are moving slowly with respect to surface clutter, heretofore these bistatic solutions have not been considered to be more than of passing interest because they are too constraining to be applied in practice. The conventional criterion requiring equal and opposite transmitter and receiver velocities is valid only for a particular transmitter-receiver-target geometry, is unduly restrictive on the motion of the receiver and is generally incapable of being satisfied in practical radar systems.
Hence, a need exists in the art for an improved system or method for detecting low Doppler targets which is capable of greater operational flexibility than prior art methods.
The need in the art is addressed by the system and method for controlling clutter Doppler spread in a bistatic radar system of the present invention, resulting in enhanced detection of low-Doppler targets in clutter or improved SAR (synthetic aperture radar) performance. In an illustrative embodiment, a bistatic radar system includes a transmitter for transmitting electromagnetic energy toward a target, a receiver adapted to receive the electromagnetic energy reflected from the target, and a processor for optimizing a parameter or parameters of the system such that the directional derivative of the bistatic Doppler field along the isorange contour is near a desired value. The parameters to be optimized may include the transmitter velocity vector, the receiver velocity vector, or the receiver azimuth flight direction. The desired value is the minimal absolute value of the directional derivative in order to minimize the clutter Doppler spread, or the maximum absolute value of the directional derivative in order to maximize the clutter Doppler spread.